ALSA: firewire-lib: remove WARN_ON() at destruction of AMDTP domain

The destructor of AMDTP domain has WARN_ON() for the list of associated
AMDTP stream. Although this reminds a case that developers forget to
program consumer drivers to stop AMDTP domain, it hits when AMDTP domain
is not initialized yet. This occurs when initialization of sound card
fails as well and it's superfluous.

This commit removes the WARN_ON. Although the API to AMDTP domain does
nothing, it's left for future usage.

Fixes: 3ec3d7a3 ("ALSA: firewire-lib: add AMDTP domain structure to handle several isoc contexts")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20190906131414.15370-1-o-takashi@sakamocchi.jpSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/amdtp-stream.c

@@ -1153,7 +1153,8 @@ EXPORT_SYMBOL_GPL(amdtp_domain_init);
  */
 void amdtp_domain_destroy(struct amdtp_domain *d)
 {
-	WARN_ON(!list_empty(&d->streams));
+	// At present nothing to do.
+	return;
 }
 EXPORT_SYMBOL_GPL(amdtp_domain_destroy);